I stumbled across this great information yesterday at the following website. This is the first time I found where someone's gathered all the information for a true compairson and description between a large selection of the IHI turbos. The reason I'm posting all the contents of the thread and the link is because I don't want all this valuable information to get lost if their website happens to go down. (we all know we have those bookmarks still saved somewhere for what reason, who knows)

IHI turbo.
The VF series (VF22, VF23, VF24, VF29, VF30) are the most common direct replacement turbos. All VF-series turbochargers use the same roller bearing, water-cooled core assembly. The differences are in their wheels and housings to achieve different flows.

The VF22 has the largest potential for peak horsepower. In other words, in the IHI model range, the VF 22 supports the highes boost levels. It is capable of running up to 25 psi. Because it is a roller bearing turbo, turbo lag is minimal...the boost comes on around 3300 rpm. Expect to max out the VF22 somewhere in the 400-450 hp range.

The VF23 starts the middle ground. It comes on boost around 3100 rpm and is capable of running 20 psi of boost. Expect to max out the VF23 somewhere in the 300-350 hp range.

The VF 24 starts to come on around 2900 rpm and will significantly improve power through the midrange over the stock TD04 turbo. However, the VF 24 is only capable of running around 17 psi.

The VF23 and VF 24 are a great replacement for those who value drivability higher than maximum power.

The VF29 & VF30 delivers a very wide increase in torque over the standard TD-series turbos. It is important to note that the VF30 is not a roller bearing turbo.

This turbo has the highest output potential of all of the IHI VF series turbos and is the best choice for those who are looking for loads of top end power. The top end power however, does not come without a cost. The VF22 spools significantly slower than the rest of the IHI models due to the larger P20 exhaust housing and is much less suited for daily driving than some of the other models. Although the largest VF series turbo, the VF22 is not quite optimal for stroked engines or those who wish to run more than 20PSI of boost.

VF23

This turbo is considered a great all-around turbo. Like the VF22 it utilizes the largest P20 exhaust housing. This housing is mated with a smaller compressor housing of the of the VF24. This turbo is considered optimal in applications with range from mild to slightly wild. It does not have the same top end power of the VF22, but spools up significantly quicker.

VF24

This turbo shares its compressor housing with the VF23 however, this housing is mated with a smaller (P18) exhaust side. The smaller characteristics of this turbo allow it to provide ample bottom end power and quick spool. This turbo is very popular for Imprezas with automatic transmissions and Group N rally cars.

VF28

This turbo came standard on the STi Version 5. In terms of overall size, it is smaller than the VF22, VF30 and VF34, and about same size as the VF23.

VF29

This Turbo is nearly identical to the VF24, with the same compressor and exhaust housings. However the compressor wheel in the VF29 is has been changed slightly. The changes made to the compressor wheel in this model are generally viewed as improvements, and as such this unit is typically chosen over the VF24.

VF30

The VF30 is commonly considered the best bang for the buck turbo in the IHI VF series line. A relatively new model the VF30 features the same exhaust housing as the VF24 but a larger compressor side similar to the VF22. The combination of these two parts results in increased output potential without the lag associated with the VF22. Although it doesn't offer the top end supremacy of the VF22, the VF30 is a great compromise between these unit and the quicker spooling models.

VF34

The VF34 is nearly identical to the VF30, with the same exhaust housing and compressor. However the VF34 goes back to the ball bearing design, and in doing so achieves full boost approximately 500RPM sooner than the comparable VF30. The VF34 is the most recent IHI design and as such costs slightly more than its counterpart. Top end performance and maximum output are identical to the 30.

VF35

VF35 The VF35 has identical internals as the VF30 and it uses divided thrust bearings. However, the exhaust housing is a P15 which means this turbo will have fantastic spool characteristics. This turbo is standard on the new WRX Type RA. LIMITED SUPPLY.

more info

VF36
Roller bearing version of the twin scroll VF37, also has a titanium turbine and shaft for even quicker spool. Same compressor housing as VF30/34, however twin scroll P25 exhaust housing provides slightly better top end output due to reduced exhaust pulse interference. This turbo is good for 400HP and used on JDM STI Spec C from 2003 onwards.

VF37 (thrust bearing)
Enter the age of twin scroll IHI turbos. Same compressor housing as VF30/34, however has a new twin scroll P25 exhaust housing that provides slightly better top end output due to reduced exhaust pulse interference. Twin scroll also provides better spool up for improved low down response over the VF30/34. This turbo is good for 400HP and used on JDM STI from 2003 onwards.

VF38
Twin scroll turbo with titanium turbine and shaft. Smaller compressor housing than VF36/VF37 provides tremendous spool up capabilities but less top end than VF36/37. The spool capabilities of this turbo are demonstrated on the JDM Legacy GT, which reaches peak torque at 2400RPM.

VF39
Single scroll turbo used on USDM STI and latest 2.5L STIs released internationally. Smaller than VF30/VF34.

VF42
Exclusive turbo to the S203/S204 models, this features a twin scroll design with a slightly larger compressor than the VF36/37 turbos and different turbine design (more blades). The VF42 is a roller-bearing turbo and is likely of similar size to the VF22 turbo, but with twin scroll exhaust housing for faster spool and superior top end performance due to reduced exhaust pulse interference.

IHI VF43
(250-325whp, Bolt-On)
This is the standard equipment turbocharger used on the MY07 USDM Subaru Impreza WRX STI. It can be found on both base STI's and STI Limited's. The VF43 utilizes a thrust bearing design and the P15 exhaust housing. The difference between the VF43 and the VF39 used previously on STI's is the size of the wastegate. The VF43 has a larger wastegate designed to reduce boost creep issues.
Expect to achieve full boost with the proper mods and a quality tune between 3000-3500rpms

IHI VF40
This is the standard equipment turbocharger used on the USDM Subaru Legacy GT. It can be found on all the current model years from 2005-2007.

IHI VF41
This is the standard equipment turbocharger used on the JDM Subaru Forester STI. It utilizes a P18 exhaust housing.

I should probably note that all the twin scroll turbos (VF36,37,38, 42) are by no means a bolt on. They require custom manifold, uppipe, and downpipe. Not to mention it's near impossible to source some of them.

i'd like to see a spectrum or graph or something of these turbos, one axis being spool RPM and one axis being flow rate or top end power

i'd rather them post concrete data then comparing one turbo to another turbo and then comparing that turbo to yet another turbo

IHI=no info.I have been searching for compressor maps for the IHI's and for some reason IHI is keeping them a secret.I mean i can find tons of Garret,Karmen & Mitsubishi maps and specs but nada on the IHI's VF series except to convert the a/r ratios of the turbine housings(divide by 25.4) They only use 3 exhaust housings (P17,P18, AND P20).

Yeah, it's hard to get anything concrete with IHI. It's even harder to find some of their turbos. Rumor has it, the VF22 is no longer available My buddy just tried to get one and that's what they told him, so he went with a 16g